Dissolving pulp is generally used in the production of viscose fibres and other textile fibres, as well as different further processed products which demand high chemical purity. Traditionally, it is required that the dissolving pulp has a high percentage of cellulose (“alpha-cellulose”) and is reactive towards various derivatising chemicals, such as carbon disulphide.
Generating high alpha-cellulose content means removing particularly the lignin and the hemicellulose material (xylan and glucomannan) from the fibres. The removal of lignin takes place in the same way as happens during the production of chemical pulp in general: by cooking lignocellulose-containing raw materials together with chemicals that remove the lignin, under conditions which are typical for each process, and which can be divided into, for example alkaline, acidic or solvent-based (organosolv) cooking methods. It is also possible to combine different cooking reaction steps which remove the lignin, referred to as multi-stage cooking. Lignin removal is completed in follow-up processes (oxygen delignification and bleaching), in which the fibre suspension generated during the cooking is treated with various lignin-removing chemicals, such as oxygen, chlorine dioxide, ozone, chlorine, hypochlorite and hydrogen peroxide.
For the removal of the hemicelluloses, many known processes have been used in the industry already for decades, which processes have been described in detail, among others, by Herbert Sixtan in his Handbook of Pulp (2006).
Alkaline extraction of sulphate pulp is also a known method, which is described in several patent applications, such as WO publication 2007/065 969.
Many of the manufacturing processes of dissolving pulp (viscose pulp) are based on sulphite cooking. Generally, in sulphite pulp mills, increasing the percentage of alpha-cellulose takes place in such a way that hemicelluloses are removed in association with bleaching, during the “hot alkali stage”, by using diluted NaOH solutions as reagents, and under conditions of high temperature.
There are also cooking processes in which it is possible to achieve the desired percentage of alpha-cellulose already during the cooking Such methods are based on multi-stage sulphite cooking or prehydrolysis sulphate cooking.
In the sulphite methods, the chemicals and especially the pH of the different cooking steps are varied appropriately. In prehydrolysis sulphate cooking, in the beginning of the process there is a special acidic hydrolysis step, in which the pH of the wood chips is brought down to a low level by using steam, water or an acid solution. Bleaching of the prehydrolysis pulp does not necessarily require a separate step at all for removing the hemicelluloses. The prehydrolysis sulphate cooking is so close to the basic sulphate process used in paper pulp production that application of it does not require a radically different technology, compared to the production of paper pulp.
A disadvantage of the prehydrolysis sulphate process is that the selectivity regarding the cellulose is compromised as the intensity of the hydrolysis is increased. This results in the consumption of wood per tonne of dissolving pulp produced becoming uneconomically high. It is possible to improve the selectivity by selecting milder conditions for the hydrolysis step, in which case not as much hemicelluloses as is theoretically possible is removed during the hydrolysis. However, a lower level of removal of hemicelluloses means that in order to achieve the desired alpha-cellulose content, the removal of hemicelluloses must be continued, for example by means of a hot-alkali-step or a cold-alkali-step, which is carried out in association with the bleaching process.
Today, the importance of hemicellulose as a biopolymer material is of increasing interest. In particular, there are new ways of using polymeric hemicellulose, for example by means of derivatisation.
As described above, traditionally the hemicellulose has had to be removed from the fibre pulp before it can be used for dissolving pulp applications. Treatments which have been carried out under hydrolysing conditions have, however, resulted in degradation of the hemicelluloses, and as a result, the percentage of polymeric material is quite small.
The publication WO 2008/098 032 describes a method, in which lignocellulose raw material, which comprises, among others, cellulose and hemicellulose, is completely dissolved in an ionic liquid. This liquid comprises almost no or only a very small amount of water. Cellulose and hemicellulose are precipitated from the ionic liquid, for example by adding water. The publication does not describe any methods of separating cellulose and hemicellulose from each other. For this reason, the regenerated lignocellulose material that is produced using this method is not pure cellulose, instead it also comprises hemicellulose. To precipitate the hemicellulose from the circulated ionic liquid, acetonitrile or THF is added into the liquid.
In an article from year 2009, Mazza et al. describe the importance of water for the solubility of cellulose in various ionic liquids (Mazza et al. Cellulose 2009, vol. 16, No. 2, pp. 207-215). The authors carried out experiments to determine how much water could be added into cellulose solution, in which the solvent was an ionic liquid. The publication makes no mention of how the water content affected the other dissolved components.
Froschauer et al., in Biomacromolecules 2012, 13, 1973-1980, describe a solvent which can be used to selectively dissolve xylan from a hemicellulose-rich pulp. They make no mention at all of the significance of the water content.